The present invention relates to a method for fabricating semiconductor memories and, in particular, to a method for fabricating a semiconductor memory using a ferroelectric capacitor as a charge storage capacitor.
Ferroelectrics typified by Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 (hereinafter, expressed as "PZT") or SrBi.sub.2 Ta.sub.2 O.sub.9 (hereinafter, expressed as "SBT"), by virtue of their having high dielectric constant and spontaneous polarization, have been being developed for applications to large capacity DRAMs and nonvolatile memories. For implementation of high density semiconductor memories using ferroelectrics, it is necessary to form stack type memory cells.
The stack type memory cell is a structure that an underlayer selective transistor and a charge-storage capacitor are connected to each other via a contact plug. When polysilicon with impurities diffused therein at high concentration is used as the contact plug (hereinafter, referred to as "polysilicon plug"), platinum, iridium, iridium oxide or the like used for a lower electrode of the ferroelectric capacitor reacts with silicon, making it impossible to obtain a stable contact resistance. Due to this, a diffusion barrier layer of titanium nitride or the like is provided so as to suppress the reaction between lower electrode and silicon.
Meanwhile, a ferroelectric film, when subjected to semiconductor fabricating process such as etching process, suffers serious damage due to the process so that its ferroelectric characteristics considerably deteriorates. For example, in dry etching process, while the substrate is exposed to charged particles, there occur various electrification phenomena, which causes deterioration of dielectric characteristics or insulating characteristics of the ferroelectric film. Further, wet etching process such as cleaning also causes deterioration of the dielectric characteristics or insulating characteristics of the ferroelectric film.
The damage due to these processes is normally recovered to the initial state by carrying out heat treatment in a high-temperature oxygen-containing atmosphere of about 500-700.degree. C. A cross-sectional structure after the processing of the lower electrode and the barrier metal is shown in FIG. 6.
In FIG. 6, reference numeral 21 denotes a silicon substrate, 22 denotes LOCOS (local oxidation of silicon) oxide for device isolation, 23 denotes gate oxide, 24 denotes a gate electrode, 25 denotes source and drain regions of a transistor, 26 denotes a first interlayer insulator, 27 denotes a polysilicon plug, 28 denotes a barrier metal, for example, a nitride of a tantalum and silicon alloy (TaSiN), 29 denotes a lower electrode, for example, iridium, 30 denotes an SBT film, which is a ferroelectric film, 31 denotes an upper platinum electrode, and 32 denotes oxidized portions of the lower electrode and the barrier metal.
In the state that the processing for up to the lower electrode and the barrier metal layer has been done, it is impossible to carry out heat treatment in a high-temperature oxygen-containing atmosphere. That is, the barrier metal of titanium nitride or tantalum nitride, tungsten nitride, TaSiN or a nitride of a titanium-silicon alloy (TiSiN) or the like and the lower electrode of iridium or the like are easily oxidized during the heat treatment in an oxygen-containing atmosphere (portions indicated by numeral 32 in FIG. 6). Therefore, the lower electrode 29 or the barrier metal 28, when subjected to heat treatment in the exposed state, is easily oxidized, incurring volume expansion or cohesion, which causes hillocks or peelings or impairment of electrical conduction between contact plug and lower electrode, thus making it impossible to carry out the heat treatment in an oxygen-containing atmosphere.
To avoid this problem, it has conventionally been practiced to carry out heat treatment in an inert gas atmosphere such as nitrogen.
However, heat treatment in an inert gas atmosphere could not allow the capacitor to recover enough. As a result, a ferroelectric capacitor obtained would be poor in electrical characteristics and exhibit unstable behavior, with the result of lower yields. In order to obtain a ferroelectric capacitor having good electrical characteristics and high reliability, it is essential to carry out heat treatment in a high-temperature oxygen-containing atmosphere without causing volume expansion or cohesion due to the oxidation of the lower electrode and the barrier metal.